The present invention relates to a single shaft combined cycle power plant start-up method. More specifically, the invention relates to a single shaft combined cycle power plant start-up method that uses steam fed from a steam source different from an exhaust heat recovery boiler to generate a torque in a steam turbine so as to start up a single shaft combined cycle power plant. Further, the present invention relates to the single shaft combined cycle power plant.
A combined power plant is a power plant that introduces exhaust heat gas of a gas turbine into an exhaust heat recovery boiler to generate high temperature, high-pressure steam and uses the generated steam as a drive source of a steam turbine. The gas turbine is basically driven by combustion gas. However, in the case of a gas turbine of a type in which a gas turbine compressor is coaxially provided, the gas turbine needs to be driven by an auxiliary start-up device until the gas turbine itself reaches a rotation speed at which a torque greater than a torque required to drive the gas turbine compressor is generated and self-sustained operation is achieved, that is, during a time period (starting from a turning operation to purge operation and subsequent ignition of the gas turbine) during which the gas turbine operates at a predetermined rotation speed.
In a single shaft combined power plant in which a gas turbine, a gas turbine compressor, a steam turbine, and a power generator are coaxially connected to one another through couplings, there have been adopted, as a start-up method of a shaft (gas turbine, gas turbine compressor, steam turbine and electric power generator are collectively referred to as “shaft”), the following methods: a method in which a cranking electric motor and a torque converter are used to drive the shaft; and a method in which a static start-up device (also referred to as “Load Commuted Inverter”) constituted by a semiconductor device such as a thyristor is used to start a power generator as a synchronous electric motor. Further, there has been adopted a method in which steam fed from an auxiliary steam boiler provided in a power plant is used to drive a steam turbine (refer to, e.g., Japanese Patent Application Laid-Open Publication No. 8-86227 (hereinafter referred to as “Patent Document 1”), the entire content of which is incorporated herein by reference).
As an example of the above method in which steam fed from an auxiliary steam boiler provided in the power plant is used to drive the steam turbine, there has recently been proposed a start-up method in which steam fed from a package boiler is fed to a high-pressure steam turbine to start a gas turbine and speed up the same to a rotating speed at which self-sustained operation of the gas turbine is achieved (refer to, e.g., Japanese Patent Application Laid-Open Publication No. 11-336510 (hereinafter referred to as “Patent Document 2”)).
As a result of investigations by the present inventors, it has been found that the start-up method of a high-pressure steam turbine in a single shaft combined power plant of the type disclosed in Patent Document 2 has the following problems.
A first problem is that plant installation cost is increased to make the power plant undesirable from an economic view point.
In a single shaft combined cycle power plant, auxiliary steam is generally used for steam turbine gland sealing, condensate deaeration, or low-pressure steam turbine cooling and, in consideration of an appropriate enthalpy and controllability of the auxiliary steam in the above use condition, the pressure and temperature of the auxiliary steam are generally set to about 0.7 MPa and 220 degrees Celsius, respectively.
In terms of economic efficiency of the plant, it is preferable to lower the pressure and temperature of the package boiler. However, in the method as disclosed in Patent Document 2 in which the auxiliary steam is introduced into the high-pressure steam turbine to start the single shaft combined cycle power plant, auxiliary steam of a higher pressure and higher temperature is required. Accordingly, in the case where the auxiliary steam is used for other purposes, the pressure and temperature thereof need to be reduced and thus a pressure reduction device and a temperature reduction device are required. The installation of the pressure reduction device and temperature reduction device leads to increased cost to adversely affect economic efficiency.
A high-pressure main steam control valve, which fully opens during the rated operation of the plant, is provided at the inlet of the high-pressure steam turbine. This valve is designed to control a large flow of high temperature, high-pressure steam and is therefore unsuitable for control of a small flow of steam performed at the time of starting-up the single shaft combined cycle power plant. Thus, in order to practice the start-up method of Patent Document 2 in the single shaft combined cycle power plant, it is necessary to provide a pressure control valve used exclusively for start-up time in addition to the high-pressure main steam control valve used during the rated operation of the plant. In this regard, economic efficiency is adversely affected.
A second problem is that it is difficult to apply a steam cooling method as a cooling method of a gas turbine high temperature part.
In the latest gas turbine, in order to increase the inlet temperature of the gas turbine for the purpose of improving plant thermal efficiency, there has been adopted a steam cooling method that uses steam with excellent heat transfer coefficient in place of an air cooling method that uses air as a medium for cooling a stator blade assembly or rotor blade assembly which is a high temperature part of the gas turbine. The present inventors have found that a use of the steam cooling type gas turbine as a component of the single shaft combined cycle power plant makes it very difficult to apply the start-up method of Patent Document 2.
That is, in the case of a single shaft combined cycle power plant in which the steam cooling method has been adopted, a method is configured such that exhaust steam of a high-pressure turbine is fed to a gas turbine cooling unit as a cooling medium and then fed to a high temperature part of the gas turbine. In the initial phase (including start-up phase and speed-up phase) of the start-up of the plant, a cooling method of the gas turbine is in an air cooling mode since steam has not yet been fed to the high-pressure steam turbine. Accordingly, air discharged from a gas turbine compressor is fed to the gas turbine cooling unit, and this air is used to cool the gas turbine. After that, when steam which is generated in the exhaust heat recovery boiler along with the progress of a start-up phase of the plant is fed to the high-pressure steam turbine to generate exhaust steam in the high-pressure steam, the cooling medium in the cooling unit changes from the discharged air of the compressor to exhaust steam of the high-pressure steam turbine, whereby the steam cooling is achieved.
However, at the initial phase of the start-up of the plant, at which the cooling method is in an air cooling mode, the pressure of the air discharged from the gas turbine compressor is in a range of about 0.7 MPa to 0.9 MPa. On the other hand, in most cases, the pressure steam condition of the auxiliary steam is set to about 0.7 MPa in terms of economic efficiency.
Assume that the high-pressure steam turbine start-up method as disclosed in Patent Document 2 is applied as the single shaft combined cycle power plant start-up method in which the steam cooling method has been adopted. In this case, the pressure of an exhaust section of the high-pressure steam turbine is held in a range of about 0.7 MPa to 0.9 MPa by the discharged air of the gas turbine compressor, if the auxiliary steam whose pressure is held at 0.7 MPa is fed to the upstream side of the high-pressure main steam control valve. Therefore, the auxiliary steam is not allowed to flow in the high-pressure steam turbine (that is, the auxiliary steam cannot be fed to the high-pressure steam turbine) in a state where the pressure relationship is reversed. Thus, the start-up device based on this assumption cannot function.
Of course, when the pressure of the auxiliary steam is increased, the problem is alleviated. However, as described above, an auxiliary boiler for supplying the high-pressure steam is generally expensive. Further, an increase in the pressure inevitably incurs an increase in the steam temperature. Thus, considering a fact that a pressure reduction device and a temperature reduction device need to be provided for multiple use purposes (e.g., for use as gland steam or deaeration steam) of the auxiliary steam, adoption of high-pressure auxiliary steam having a pressure far exceeding 0.7 MPa is not practical.
A third problem is that in the case where hot start-up is performed during DSS (Daily Start-up & Stop) operation, reverse mismatch phenomenon occurs between the metal temperature of the high-pressure steam turbine and auxiliary steam temperature.
The single shaft combined cycle power plant including not only a single shaft combined cycle power plant of a steam cooling type but also a single shaft combined cycle power plant of an air cooling type can start/stop at short times and therefore can be operated under DSS in which the plant is started and stopped on a per day basis. In the case where hot start-up is performed, that is, in the case where the plant is restarted within a comparatively short time period after the stop of the turbine, the metal temperature has been decreased to a range of about 400 degrees Celsius to 500 degrees Celsius, while the temperature of the auxiliary steam whose pressure is held at 0.7 MPa is no more than about 230 degrees Celsius.
At this time, reverse mismatch state in which the metal temperature is higher than the temperature of the auxiliary temperature flowing in the high-pressure steam turbine is established. If the hot start-up is performed in this reverse mismatch state, the steam turbine may be broken. Therefore, the high-pressure steam turbine start-up method is not suitable for the hot start-up. This imposes a fatal operational restriction on the single shaft combined cycle power plant that is easily ready for operation under DSS.